A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head, which is housed in a slider, and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies in part on a suspension's force on the slider and on the aerodynamic characteristics of the slider air bearing surface (ABS) to provide the proper distance between the read/write head and the surface of the magnetic-recording disk (the “flying height”) while the magnetic-recording disk rotates.
Increasing areal density (a measure of the quantity of information bits that can be stored on a given area of disk surface) has led to the necessary development and implementation of secondary and even tertiary actuators for improved head positioning through relatively fine positioning, in addition to a primary voice coil motor (VCM) actuator which provides relatively coarse positioning. Some hard disk drives employ micro- or milli-actuator designs to provide second and/or third stage actuation of the recording head to enable more accurate positioning of the head relative to the recording track. Milli-actuators are broadly classified as actuators that move the entire front end of the suspension: spring, load beam, flexure and slider, and are typically used as second stage actuators. Microactuators are typically used as third stage actuators and are broadly classified as actuators that move only the slider, moving it relative to the suspension and load beam, or move only the read-write element relative to the slider body. A third stage actuator is used in conjunction with a first stage actuator (e.g., VCM) and a second stage actuator (e.g., milli-actuator) for more accurate head positioning.
Piezoelectric (PZT) based and capacitive micro-machined transducers are two types of microactuators that have been proposed for use with HDD sliders. Typical PZT transducers are designed as two cantilevers joined with a rigid body at the free ends, a structure which leads to a modestly high bandwidth of 20 kHz but which is very complicated to manufacture because the two cantilevers are fabricated using sand blasting techniques. In the case of capacitive micro-machined microactuators, the net displacement that can be achieved is relatively high but is offset in part by a low servo bandwidth due to a relatively low resonant frequency, often in the range of 5 kHz.